Magnesium serves as the dominant p-type dopant species in GaN and its alloys, and it is typically delivered during metalorganic chemical vapor deposition (MOCVD) via the organometallic precursor magnesocene (MgCp2, Cp=cyclopentadienyl). Unfortunately, controlled incorporation of Mg into III-nitride films via MOCVD has not been without serious problems. Frequently, unpredictable Mg concentration profiles occur that can vary greatly after the reactor has been perturbed (for example, from any repair) or even between sequential wafer-growth runs. Even in reactors in which the Mg doping process is largely reproducible, the Mg profile in the grown films is marked by gradients corresponding to the turn-on and turn off of the doping process. This behavior, commonly referred to as a Mg “memory effect,” is problematic since abrupt doping profiles rather than doping gradients are often required for optimal device performance.
The Mg doping response was reported for GaN MOCVD in Y. Ohba and A. Hatano, “A study on strong memory effects for Mg doping in GaN metalorganic chemical vapor deposition,” J. Crystal Growth 145 (1994) p. 214-218. A delay time was observed between Mg atom incorporation into the growing layer and the supplying of the Mg source to the reactor, in spite of the quick response for doping turn off. The delay time was considered to originate from adsorption of Mg source molecules on the reactor wall.
Redistribution of Mg into a subsequently regrown GaN epilayer grown by MOCVD has been studied (H. Xing et al, “Memory Effect and Redistribution of Mg into Sequentially Regrown GaN Layer by Metalorganic Chemical Vapor Deposition,” Jpn. J. Appl. Phys. 42 (2003) p. 50-53.) The regrowth study using a Mg-free reactor revealed that a Mg-rich film was present on MOCVD as-grown GaN:Mg base layers that could be removed by an acid etch, and that a slow Mg decay into the sequentially regrown GaN resulted from the Mg-rich surface film.
Memory effects have been seen in the growth of sequential layers in a single wafer. Secondary-ion mass spectroscopy (SIMS) has been used to study the Mg concentration in a sample with sequentially grown layers of undoped and Mg-doped GaN (J. Schurman et al, “Reproducibility of GaN and InGaN films grown in a multi-wafer rotating-disc reactor,” Mater. Sci. Engin. B43 (1997) p. 222-227). For each layer, the Mg concentration started out low and slowly increased during the growth of the layer. Also of interest was the increase in the background Mg concentration between each Mg-doped layer. A sample was grown with sequentially grown layers of Mg- and Si-doped GaN. The Mg background concentration is seen to increase between Mg layers. The overall level of Mg in the reactor slowly decayed after the growth of the last Mg-doped layer. This was evident from the slow drop in the Mg level in the top Si-doped layer of the structure.